The present invention relates to a method in which specimens are acted on by a modulated beam of light and the variations in density which are induced thermally as a result of absorption of the light are detected optically at the tempo of the modulation frequency by a second beam of light.
This method is closely related to so-called photoacoustic spectroscopy (PAS) in which the dependence of the absorption of the wavelength of the irradiated light, determined acoustically via variations in density, is investigated. It is furthermore known to produce, on the basis of this method of photoacoustic spectroscopy, an image display of absorbing regions of the specimen within which the exciting beam of light is guided with fixed wavelength over the specimen and the intensity of the detection signal is reported as a function of the locus in the specimen.
For the detection of the variations in density, PAS (photoacoustic spectroscopy) employs a microphone. This microphone may be contained in a pressure-proof housing together with the sample to be examined, and takes up the acoustic waves produced in the gas-filled interior of the housing. Alternatively, the microphone may be cemented directly to the housing and detects the solid-conducted sound which is conducted by the sample itself. Such methods and apparatus are described in Analen der Chemie 47, (1975), page 592 and in British patent application document GB No. 2047896 A, issued 3, Dec. 1980.
Furthermore, from Applied Physics Letters 36 (2), 1980, pages 130-132, a method of detecting variations in density is also known which consists of conducting a beam of light along and slightly above the surface of the specimen to be examined, and detecting the cyclic variations in the angular deflection of the beam as a result of the refractive-index gradients developed over the specimen. By means of this optical method of detection which is based on the so-called mirage effect, sensitivities comparable to those of acoustic detection can be obtained. Since it is not necessary to have a closed specimen chamber and further direct handling of the specimen to be examined (cementing of a piezoelectric sensor) is not necessary, the last-mentioned optical method of detection can be easily handled.